1. Field of the Invention
The invention relates to power transmission output speed control devices generally, and more specifically, to speed control of turbine type pumps requiring a relatively small range of speed variability.
2. Description of the Prior Art
In the prior art, various arrangements are extant for speed control of the type required in the turbine pump application. A typical turbine type pump is a vertical shaft unit with an induction electric motor driving the shaft. At a remote end of the shaft, down into a well casing, for example, a pump bowl assembly forces water upward. In well-known water systems, the turbine bowls and the drive motor are matched to fixed output water pressure and volume requirements and constant speed operation is appropriate.
In some applications, such as in irrigation systems, for example, water demand rates are variable and accordingly some form of water pump speed variability must be efficiently achieved. Of course, water pump output can simply be throttled back by partial valve closing to increase the effective head against which the pump operates to reduce flow. However, this option is wasteful of electric power. At current electric energy cost the resulting inefficiency is economically unacceptable.
Variable speed transmissions are well-known in the prior art, many of these being of the automotive type designed to accommodate a large range of engine speed variation and a similarly large range of output torque requirements. Such transmissions commonly include torque converters and/or step-wise gear shifting. Certain compromises among efficiency, engine speed modulation and output torque are necessary and, accordingly, these transmissions are not applicable to the limited speed variation range, high efficiency requirement inherent in irrigation system pumping. U.S. Pat. Nos. 3,903,756 and 4,836,049 are representative of that prior art.
A device including a fluid torque converter and a differential gearing arrangement for a variable speed processing machine is described in U.S. Pat. No. 4,726,255. That device is not suitable for application to the water pumping environment for which the present invention is devised.
Hydraulic transmission units are well-known in the art and have been extensively used in variable speed transmissions. However, they are commonly employed in series with the primary power train in a variable transmission system. These known elements are used in both fixed and variable displacement types and typically can function as hydraulic pumps or as hydraulic motors U.S. Pat. Nos. 4,750,381 and 4,813,306 are typical of that art.
Various other gear shifting, torque converter and clutching combinations are also extant. U.S. Pat. Nos. 4,295,792; 4,432,212 and 3,924,489 are representative of such combinations.
A belt and pulley drive with variable speed control by means of a variable pitch pulley is described in U.S. Pat. No. 4,381,174. Slip-clutch devices have been used for the purpose, but do not fulfill the power conservation objectives because of their poor efficiency.
The aforementioned examples of the prior art disclose system each for a particular purpose, but none of them would be structurally or functionally suitable for the application served by the invention disclosed hereinafter.
Perhaps the most pertinent prior art is that of U.S. Pat. No. 4,679,462 which employs some of the same components. However, that reference contemplates summation of the variable input power (from a variable speed aircraft engine) in a mechanical differential with a trim torque developed and applied to keep the output shaft speed constant for driving a generator or alternator at constant speed. The invention, on the other hand, contemplates a trim power take off from a differential and feeding the resultant torque back to the main power shaft either between the motor (main power source) and the differential or between the differential and the turbine pump shaft via a hydraulic pump/motor combination.
Another approach to variable speed turbine pump drive includes an A.C. line frequency changer such that the driving motor is caused to operate at a speed other than its 60 cycle design speed. Normally, the induction motor used operates at 1760 R.P.M. just slightly under synchronous speed of 1800 R.P.M. for a four pole motor. That motor speed is variable with load in a minor way, but for present purposes may be considered to be a constant speed power source. Varying the A.C. line frequency proportionally changes the motor speed, however, this being accomplished by rectification of the 60 cycle A.C. line source and feeding through a variable frequency inverter to set motor speed accordingly. Such a system is effective but it is noted that all the induction motor power must be handled by the rectifier and inverter. Accordingly, the power losses thus introduced are substantial and the cost of such a system is relatively large.
The manner in which the invention deals with the disadvantages of the prior art in setting forth a novel, efficient and relatively low cost system will be described hereinafter.